# Field Sobriety Test
**URL:** https://boards.straightdope.com/t/field-sobriety-test/310961
**Category:** Factual Questions
**Created:** [July 3, 2005, 11:11pm UTC](https://boards.straightdope.com/t/field-sobriety-test/310961 "2005-07-03T23:11:34Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@Argo84](https://boards.straightdope.com/u/Argo84)
#### Post date: [July 3, 2005, 11:11pm UTC](https://boards.straightdope.com/t/field-sobriety-test/310961/1 "2005-07-03T23:11:34Z")
A common test used by police officers conducting field sobriety tests is to have a suspect hold his/her head still and follow a pen tip with their eyes. What are the officers looking for? What do your eyes do while you’re under the influence that they don’t do when you’re not?
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### Author:  [@Scruloose](https://boards.straightdope.com/u/Scruloose)
#### Post date: [July 3, 2005, 11:20pm UTC](https://boards.straightdope.com/t/field-sobriety-test/310961/2 "2005-07-03T23:20:42Z")
It’s the “Horizontal Gaze Nystagmus” test. Basically, your eye movements react differently under the influence of alcohol. Seasoned cops can get a pretty accurate guage of one’s BAC level with this test. (so I’m told) There is also a “Vertical Gaze” test for a few types of narcotics.
[HGN](http://www.nhtsa.dot.gov/people/injury/enforce/nystagmus/hgntxt.html)
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### Author:  [@St.Urho](https://boards.straightdope.com/u/St.Urho)
#### Post date: [July 4, 2005, 1:10pm UTC](https://boards.straightdope.com/t/field-sobriety-test/310961/3 "2005-07-04T13:10:45Z")
A person’s BAC can be determined by this formula
```auto
50 - angle of onset = B.A.C.
```
This can also be used to support the breathalyzer reading. Vertical gaze nystagmus typically occurs with very high blood alcohol levels, CNS depressant use, inhalant use, and PCP use.
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### Author:  [@St.Urho](https://boards.straightdope.com/u/St.Urho)
#### Post date: [July 4, 2005, 1:12pm UTC](https://boards.straightdope.com/t/field-sobriety-test/310961/4 "2005-07-04T13:12:17Z")
Hmm, maybe I’ll actually finish that post. So if someone had an onset of nystagmus at 40 degrees, their blood alcohol would be estimated at .10